Radiant heat-induced hyperthermia in mice: in vivo effects on the immune system.

Whole-body hyperthermia (WBH) in mice was induced by 2-4-h exposure to radiant heat resulting in core body temperatures of 38.5-40.4 degrees C, and correlated directly with the magnitude and duration of heat treatment. Two-hour heat treatments in this temperature range did not consistently affect generation of antibody-forming cells in vivo, while 4-h treatments at temperatures greater than or equal to 40 degrees C significantly suppressed the antibody-forming cell response. The capacity of lymphocytes from similarly heated mice to generate antibody-forming cells in vitro was not affected, suggesting that the observed in vivo suppression may be mediated by circulating factors rather than by some heat-induced alteration in the cells themselves. In vivo treatment did not alter T-cell responsiveness to the mitogen Con-A or delayed-type hypersensitivity responses to sheep red blood cells. Quantitative and flow cytometric assessment of splenic and thymic lymphocyte numbers showed that WBH did not alter absolute numbers of lymphocytes but did temporarily change the proportions of lymphocyte subsets. An immediate increase in splenic L3T4+ cells was observed, followed within 18 h by an overall decrease in Lyt 2+ and Thy 1.2+ T-cells. In the thymus the percentages of mature T cells increased. In general, only minimal effects of heat on the immune responses of normal mice could be demonstrated.